Element having phosphine activated photosensitive compositions therein

ABSTRACT

A photopolymerizable composition and photosensitive elements produced therefrom are disclosed. The photopolymerizable composition includes: (a) a monomer component, (b) a polymer component, (c) a photopolymerization initiator, and (d) an activator. 
     The monomer component contains at least one water soluble, monofunctional unsaturated ethylenic monomer or the combination of such a monomer and at least one polyfunctional unsaturated ethylenic monomer. 
     The polymer component includes a partially saponified water soluble, polyvinyl acetate polymer compatible with the monomer components. The polyvinyl acetate polymer contains both acetyl and hydroxyl groups and has a polymerization degree of from 300-2000. The degree of saponification of the polymer is desirably in the range of 65 to 99 percent. 
     The preferred photopolymerizaton initiators are acetophenone derivatives such as 2,2-dimethoxy-2-phenyl acetopheonone, 2,2-diethoxy-2-phenyl acetophenone, 2,-2-diisopropoxy-2-phenyl acetophenone, benzoin alkyl, aryl, allyl or vinyl ethers, or benzophenone derivatives such as benzophenone, 2-methylbenzophenone, 2-methoxybenzophenone or 2,2&#39;-dimethoxybenzophenone, which are compatible with the monomer component and the polyvinyl acetate polymer and are activatable by actinic light. 
     Finally, the activator is a phosphine derivative generally described by the formula: ##STR1## wherein any of X, Y and Z are hydrogen, halogen, alkyl, alkoxy, aryl, vinyl, or allyl, although not more than one of X, Y and Z may be hydrogen. It is especially preferred in the practice of the present invention that at least one of X, Y or Z in the phosphine derivative activator component be an aryl group.

RELATED APPLICATION

This is a division of application Ser. No. 70,190, filed Aug. 27, 1979,now U.S. Pat. No. 4,233,391 which is a continuation-in-part ofapplication Ser. No. 941,519, filed Sept. 9, 1978, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to a new class of water-developablephotopolymerizable compositions, and photosensitive elements andphotopolymer printing plates that utilize such compositions. Morespecifically, the novel photopolymerizable compositions of the presentinvention can be used without any pre-exposure conditioning of the typethat has become customary with various prior art compositions. Thepresent compositions thus provide photosensitive elements andphotopolymer printing plates that not only have vastly improved printingcharacteristics, but can be directly subjected to an imagewise exposureand then developed with water without any costly or time consumingpre-exposure conditioning steps.

Many photopolymerizable compositions and photosensitive elements thatare useful in making printing plates of one kind or another are known inthe art. One, if not the most, commercially successful of all such priorart compositions is described in U.S. Pat. No. 3,801,328 to Takimoto etal. Such compositions, although they have met with enormous commercialsuccess, nevertheless require some form of pre-exposure conditioningbefore imagewise exposure in order to produce commercially satisfactoryrelief images on printing plates that employ such compositions.

Two techniques for pre-exposure conditioning have evolved in the art.The first, commonly known as CO₂ conditioning, requires the maintenanceof the photosensitive element or plate in a CO₂ atmosphere until justprior to the imagewise exposure of the element or plate. The secondknown technique, which has supplanted the CO₂ conditioning technique andis commonly known as "bump exposure", requires that the photosensitiveelement or plate be subjected to a brief (typically 2 to 8 second) lightexposure immediately prior to the longer duration imagewise exposure, inthe presence of a negative.

It has been recognized, of course, that both pre-exposure conditioningtechniques require special, costly and time consuming additionalhandling of the photosensitive element or plate that, from a plateprocessing standpoint should desirably be eliminated. Moreover, bothtechniques make the design and operation of automatic plate exposure andprocessing equipment unduly complicated and expensive. And perhaps evenmore importantly, both techniques, because of the necessity thatimagewise exposure be carried out within a designated time period afterpre-exposure conditioning occurs, introduce time controls in plateprocessing that are oftentimes difficult, if not impossible, tocarefully maintain in the large quantity, commercial newspaperapplications for such plates.

It has been determined by the applicants herein, for example, that aprinting plate that has been "bump" exposed should desirably besubjected to an imagewise exposure within approximately 30 seconds afterthe bump exposure is completed. Variations in this period of platedormancy, between the completion of the bump exposure step and theinitiation of the imagewise exposure steps can, and often does, producedeveloped plates having vastly different printing qualitycharacteristics. Thus, two plates utilizing the same photopolymercomposition and same negative can have different printingcharacteristics simply because the period of dormancy between bump andimagewise exposure varies.

Moreover, photosensitive elements and plates, which are subjected tobump exposure, must typically be exposed under high intensity pointsources of actinic light, such as carbon are lamps or high pressuremercury vapor lamps. Such lamps not only have extremely high currentdemands and short lives, but in normal operation produce unwanted heat,which can cause or contribute to the degradation of thephotopolymerizable compositions or cause negatives to stick to the platephotopolymer surface.

As a result of the many inconveniences, the added expenses andvariations in product quality inherent in the use of plates requiringsome form of pre-conditioning, either through CO₂ conditioning or bumpexposure, a need has developed for water-developable photopolymerizablecompositions (and resultant elements and plates) that entirely eliminatethe necessity of pre-exposure conditioning, yet at the same time provideall of the advantageous properties of the commercially desirablecompositions of the type described in the Takimoto U.S. Pat. No.3,801,328 patent. One such effort to solve the pre-exposure conditioningproblem is described in U.S. Pat. No. 4,042,386 to Okai and Kimoto,wherein the selection and use of specific components, in certain ratiosand amounts, and intensified exposure of the resulting photosensitiveelement or plate, are disclosed as means of eliminating the need forpre-exposure conditioning. This approach, however, suffers from thedisadvantage that only carefully selected components and ratios amountsof such components can be used, and from the additional disadvantagethat only costly high intensity light sources (which have shortened lamplives and generate excessive amounts of undesired heat) can be used.Moreover, it has been determined that the photopolymerizablecompositions disclosed in the Okai and Kimoto patent work best withlower reflectivity substrates (e.g., a high degree of antihalation) inorder to provide developed photopolymerizable elements having thedesired dot depth characteristics in highlight, middle tone and shadowimage areas.

SUMMARY OF THE INVENTION

The photopolymerizable compositions of the present invention not onlyeliminate the need for any pre-exposure or CO₂ conditioning of theresultant photosensitive elements and plates that employ suchcompositions, but provide water-developable elements and plates that canutilize a wide range of monomer and polymer components, requireshortened exposure times and less antihalation conditioning of the platesubstrate, can be exposed with less intense light sources, and yet stillprovide consistently improved printing characteristics; e.g., improveddot depth in highlight, middle tone and shadow image areas, and improvedminimum line width and shapes in reverse areas. Furthermore, thephotopolymerizable compositions of the present invention can be appliedto ultra-shallow relief plates, such as 4 mils to 8 mils in reliefthickness, because such compositions are not adversely affected byoxygen exposure.

The surprising and unexpected properties of the photopolymerizablecompositions (and resulting photopolymer elements and plates) of thepresent invention are the result of the selection and use of a uniquecombination of components, including a defined class of phosphinederivative activator components, which interact with the remainingphotopolymerizable composition components to provide the heretoforeunrecognizable and unachievable advantages of the present invention.

It is, therefore, an object of the present invention to provide a newphotopolymerizable composition and a new photosensitive element which donot require pre-exposure conditioning.

It is another object of the present invention to provide aphotopolymerizable composition and a photosensitive element whichrequire shorter exposure times than is required with prior compositions.

It is yet another object of the present invention to provide aphotopolymerizable composition and a photosensitive element which can beexposed by a non-diffusion actinic light source.

It is still a further object of the present invention to provide aphotopolymerizable composition and a photosensitive element which can beprocessed more quickly and more efficiently than was hitherto possible.

These and other objects of the present invention together with itsvarious advantages will become apparent to those skilled in the art froma consideration of the following detailed description of variousembodiments of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The photopolymerizable composition of the present invention generallyincludes the following four components:

A. A monomer component including at least one water-soluble,monofunctional unsaturated ethylenic monomer, or the combination of sucha monomer and at least one polyfunctional unsaturated ethylenic monomer,said monomers being capable of forming a polymer by photoinitiatedpolymerization in the presence of a polymerization initiator activatableby actinic light;

B. A polymer component including a partially saponified, water-soluble,polyvinyl acetate polymer compatible with said monomer component,containing both acetyl and hydroxy groups, and having a polymerizationdegree of about 300 to 2,000, and a saponification degree of about 65 to99 mole percent;

C. A photopolymerization initiator compatible with said monomercomponent and said polyvinyl acetate polymer, and activatable by actiniclight; and

D. An activator component including a phosphine derivative described bythe formula: ##STR2## wherein any of X, Y and Z are hydrogen, halogen,alkyl, alkoxy, aryl, vinyl, or allyl, and wherein not more than one ofX, Y and Z may be hydrogen.

It has been found especially desirable in the practice of the presentinvention that at least one of X, Y or Z in the phosphine derivativeactivator component be an aryl group. Indeed, in those instances inwhich a phosphine derivative without at least one aryl group have beenused (such as tri-n-butylphosphine) the resultant photopolymer plateshave unacceptable properties. Thus, the phosphine derivative ispreferably selected from among the following compounds: triphenylphosphine, o-tolyl diphenyl phosphine, di-(o-tolyl) phenyl phosphine,tri-(o-tolyl) phosphine, o-methoxyphenyl diphenyl phosphine,o-ethylphenyl diphenyl phosphine, and o-chlorophenyl diphenyl phosphine.Most preferable among those phosphine derivatives is triphenyl phosphinebecause of its improved sensitivity, cost and air stability. As notedabove, the absence of at least one phenyl group in the phosphineactivator component of the present invention does not eliminate thepre-exposure conditioning step. Similarly, the presence of at least onephenyl group (and preferably two or more phenyl groups), especially inan air stable form provide the correct balance of printing plateproperties, the elimination of any pre-exposure conditioning and thereduced exposure time and other advantages of the present invention.

The monomer component A found useful in the practice of this inventiongenerally comprises: (1) a monofunctional acrylic or methacrylic esterof a lower alkanol having one or more hydroxy groups, or (2) thecombination of such a monofunctional monomer and a polyfunctionalacrylic or methacrylic ester of polyethylene glycol [HO(CH₂ CH₂ O)_(n)H] which is etherified or esterified at one end thereof and wherein n isdesirably between one and twenty-three. Examples of the monofunctionalunsaturated ethylenic compound are B-hydroxyethel acrylate,B-hydroxyethyl methacrylate and B-hydroxypropyl acrylate. Examples ofthe polyfunctional monomer are polyethylene glycoldiesters (e.g.,polyethylene glycol diacrylate or polyethylene glycol dimethacrylate).In the said polyethylene glycol derivatives, n is one to twenty-three.In addition, other monomer components such as pentarerythritoldimethacrylate, trimethylolpropane trimethacrylate, tetramethylolmethanetetramethacrylate, and trimethylolethane trimethacrylate may be usedsolely or in combination.

It should be recognized that the ethylenic unsaturated monomer componentincludes mixtures of both mono- and polyfunctional compounds, themonofunctional compounds serving as solubilizing materials for theinitiator component, and the polyfunctional compounds enhancing theadhesive characteristics of the resultant photopolymerizablecomposition. Thus, by using both the water-soluble monofunctionalethylene unsaturated compound and polyfunctional ethylenic unsaturatedcompound, the highly desirable balance of properties, e.g.,water-washability and high adhesion, can be imparted to thephotopolymerizable composition.

The polymer component B used in the practice of this invention is apartially saponified polyvinyl acetate, preferably having an averagedegree of polymerization of 300 to 2,000 and a saponification degree of65 to 99 mole percent. If a suitable partially saponified polyvinylacetate cannot be obtained by saponifying polyvinyl acetate having a lowsaponification degree as a homopolymer, a copolymer obtained, forexample, by copolymerizing vinyl acetate with maleic anhydride can bepartially saponified to give the desired polymer. Saponification as usedherein is intended to mean the conversion of ester groups or the likeinto alcohol groups and the saponification degree represents the extentto which ester groups or the like have been converted to alcohol orhydroxy groups.

The hardness of the obtained printing plate as well as the speed ofwashing out in the developing procedure depends directly on the degreeof saponification. Thus to accomplish the desirable balance ofproperties in a finished developed plate, it is preferred that thepartially saponified polyvinyl acetate have a degree of saponificationwithin the stated range in saponifying polyvinyl acetate. There are somecases, for example, wherein a certain degree of saponification isrequired for compatibility with certain monomer components. Thus, it hasbeen recognized that the required degree of saponification may beobtained by mixing two or more partially saponified polyvinyl acetatepolymers having different degrees of saponification and calculating thearithmetic average of the different degrees of saponification. Mixturesof two or more polymers can be used, therefore, each having a differentsaponification degree, to obtain the desired average saponificationdegree for any given monomer component. This feature of the presentinvention adds significant flexibility to the formulation process andcontributes significantly to the balance of properties that are desiredfor the photopolymerizable composition.

As the initiator component C, acetophenone, benzophenone or benzoinderivatives are used. Benzoin derivatives are soluble in the ethylenicunsaturated monomer component and are highly compatible with the othercomponents in the aqueous composition. Moreover, since benzoinderivatives are not decomposed thermally below about 100° C., they donot harden or become insoluble with heating during the preparation ofthe photopolymerizable composition. Specific examples of the benzoinderivatives found useful in the practice of this invention are methyl,ethyl, isopropyl, isobutyl, octyl, vinyl, aryl and allyl ethers ofbenzoin, i.e., benzoin methyl ether, benzoin ethyl ether, benzoin vinylether benzoin allyl ether, etc. The benzophenone derivatives found usedin the practice of the present invention include benzophenone,2-methylbenzophenone, 2-methoxybenzophenone, 2,2-dimethoxybenzophe andthe like. The presently preferred initiators, however, are acetophenonederivatives, such as 2,2-dimethoxy-2-phenyl acetophenone,2,2-diethoxy-2-phenyl acetophenone, 2,2-diisopropoxy-2-phenylacetophenone, and 2,2-di-n-butoxy-2-phenyl acetophenone, because oftheir unexpectedly high solubility in the monomer component of thephotopolymerizable compositions and their excellent photoinitiationproperties.

Although ordinarily photopolymerization requires a photoinitiator suchas an anthraquinone compound in addition to the polymerizationinitiators discussed above, a high photosensibility can be attained inthe present invention by using acetophenone or benzoin derivativeinitiators alone.

The amounts of the various components of the photopolymerizablecomposition of the present invention should preferably lie within thefollowing weight ratios based on the total weight of the composition:(a) about 0.1 to 3.0 parts by weight of the monomer component, (b) about0.1 to 3.0 parts by weight of the polymer component, (c) about 0.001 to0.3 parts by weight of the polymerization initiator, and (d) about 0.001to 0.3 parts by weight of the activator component. It should berecognized, however, that these are suggested ratios which have beenexperimentally determined to be desirable and preferred. These ratioscan nonetheless be altered depending on the particular uses contemplatedfor the printing elements to be made using the compositions of thepresent invention. It has been found, however, that when theconcentration of the phosphine derivatives used in the present inventionexceeds the broad concentration ranges set forth above, the lightsensitivity of the photopolymerizable composition is so enhanced as tomake it very difficult to handle the photosensitive element prior toimagewise exposure. Also, excessive amounts of the phosphine derivativescause the photopolymerizable composition to become heat sensitive whichfurther interferes with the practical use of the photosensitive elementsand reduces their useful shelf life.

When, on the other hand, the concentration of the phosphine derivativeused is less than that specified in the broad concentration rangedescribed above, the sensitivity of the photosensitive element willgenerally be too low to be practically used without the undesiredpre-exposure conditioning.

It is also desirable in some instances to add to the photopolymerizablecomposition of this invention about 0.01 to 0.3 percent, based upon theamount of the monomer component, of a thermal polymerization inhibitor.

Among the thermal polymerization inhibitors found useful in the practiceof this invention are 2,6-di-t-butyl-p-cresol, hydro quinone, andp-methoxyphenol. The preferred inhibitor is 2,6-di-t-butyl-p-cresolbecause of its compatibility with the other components.

The novel photopolymerizable composition of the invention can be castdirectly on a metallic and/or plastic supporting adhesive layer. Smallamounts of dyestuffs may be added to the photopolymerizable compositionto provide antihalation properties. Generally, sufficient amounts areadded to the photopolymerizable composition just below that at which thecomposition becomes hazy. Examples of an effective dyestuff are rosebengal, eosine, methylene blue, and malachite green. These dyestuffs maybe used solely or in combination in a ratio of 20 and to 150 ppm basedon the amount of the photopolymerizable composition.

The metallic and/or plastic substrate used can be either chemically orphysically treated prior to coating with the photopolymerizable materialin order to insure strong adhesion with the photopolymerizablecomposition. This roughening or grating of the substrate surfaceeliminates the need for a separate adhesive layer although such may beused, where desired, in certain instances.

The preferred substrate is a metallic plate and preferably an aluminumor tin plate. As described in U.S. Pat. No. 3,877,939, the use of adispersed antihalation material can be eliminated by treating thesubstrate with an aqueous solution of a chromic compound. Thus, forexample, an aluminum plate would first be abraded and then chemicallytreated to impart the desired antihalation properties. The preferredchemical treatment for obtaining these antihalation properties is achemical solution of a chromic compound which includes, for example,chromic acid, water soluble salts of chromic acid and ferricyanic orferrocyanic acid and water soluble salts thereof.

Generally, the useful treating compositions contain either chromic acidor sodium bichromate, as a required component, and mixtures of potassiumferricyanide, barium fluosilicate, sodium ferrocyanide, sodiumfluoborate, potassium fluozirconate and related compositions. Thepresently preferred treating compositions is 2 percent aqueous solutionof "Alodine 1200" manufactured by Amchem Products, Inc., Ambler, Pa. Thetreating composition is typically first placed in a bath and then thealuminum or tin support plate is immersed in the bath for approximately40 seconds. After the support plate has been treated in this fashion, itgenerally changes its color from the metallic aluminum or tin color toan orange color. In this way, the orange-colored aluminum or tin supportplate itself provides the antihalation function for the resultantphotopolymer printing plate.

As discussed above, however, the degree of antihalation required in thephotopolymer plates made from the photopolymerizable plates made fromthe photopolymerizable compositions of the present invention is reduced,since the presence of the phosphine derivative activator component inthe composition minimizes the need for lower reflectivity substrates,and thus reduces the manufacturing cost of the photopolymerizablecompositions.

Moreover, the exposure of a photosensitive element, including a layer ofthe water developable photopolymerizable composition described hereinupon a substrate involves the step of subjecting the element to a singleimagewise exposure of actinic light. Although prior photosensitiveelements required the use of high intensity point sources such as carbonarc lamps and/or high pressure mercury lamps, exposure of thephotopolymerizable composition of the present invention may be carriedout under diffusion light sources such as black light tubes, otherwiseknown as chemical lamps. These chemical lamps are far less expensivethan the prior high intensity point sources, use considerably lessenergy, and generate less heat. Such chemical lamps give off actiniclight with wavelengths in the desired range of 300-400 nanometers, andprovide the further advantage of generating better shoulders in thefinal relief image which results in improved resolution and highlightreproduction.

By eliminating the previously required pre-exposure conditioning that ischaracteristic of prior art photopolymerizable compositions, thecompositions of the present invention shorten plate processing time, andare more readily useable in automatic plate developing equipment. It isno longer found necessary, for example, to engage in the time consumingtask of first bump exposing a plate, and then placing a negative on theplate for imagewise exposure. Instead, a negative can be immediatelyplaced on the plate (without bump exposure) and then exposed for ashorter period of time with less intense light, without sacrificingprinting plate quality in terms of dot depth or minimum line size.

It should be understood that printing plates having varying degrees ofphotopolymer thickness can employ the photopolymerizable compositions ofthe present invention, including relief plates having photopolymerthicknesses greater than 0.020 inch and shallow relief plates havingphotopolymer layers between 0.004 and 0.020 inch in thicknesses. Whensuch shallow relief plates are desired a binder composition isinterposed between the substrate and photopolymer layer. The bindercomposition contains dispersed within its particles which are present ina size and concentration sufficient to create a plurality ofprotuberances in the background areas of the photosensitive elementafter its exposure and development to create a relief image. Usefulbinders include polyesters, polyurethanes, polyethylene-butadienecopolymers, polyvinyl acetate derivatives, polyamides, epoxy resins,styrene-butadiene copolymers, mixtures of such copolymers and partiallyhydrolyzed polyvinyl acetate, unsaturated polyesters made, for example,from diethylene glycol, maleic anhydride and phthalic anhydride,mixtures of such polyesters and partially hydrolyzed polyvinyl acetate,and mixtures of glyoxal and partially hydrolyzed polyvinyl acetate. Thedispersed particles suitable for use in such plates include glass,"Teflon" polytetrafluroethylene, and alumina beads. Particle size andconcentration will of course be chosen on a case by case basis dependingon the printing parameters such as the thickness of the relief image,printing pressures, number of repetitions, etc.

Presently preferred and practical embodiments of the present inventionare illustrated in the following examples wherein all percentages andparts are by weight unless otherwise indicated.

EXAMPLE 1

Substrates of the photosensitive elements used below are prepared asfollows:

Method A: An aluminum plate is immersed in a surface treating agentcomprising sodium bichromate (1.0 part), concentrated sulfuric acid (10parts) and water (30 parts) at 70° to 80° C. for about 20 minutes. Then,the plate is washed with water and dried.

Method B: A tin plate is immersed in a detergent comprising Ridoline(1.0 part; manufactured by Amchem Products, Inc.) and water (30 parts)at 65° C. for about 20 minutes. Then, the plate is washed with water anddried.

Method C: The solution (70 parts) which consists of 14 parts ofpartially saponified polyvinyl acetate (average polymerization degree,500; saponification degree, 78.0 mole %) and 86 parts of water, and thewell-ground pigment paste (10 parts) which consists of 14 parts ofabove-mentioned polyvinyl acetate, 56 parts of water and 30 parts of rediron oxide pigment (RO-5097 manufactured by Pfizer) are mixed together.The latex of carboxylated styrene/butadiene copolymer (20 parts) (XD-655manufactured by Dow Chemical Co.) is added into the resulted solutiongradually under stirring. The solution is cast on the plate which isdescribed in Method B and dried for 2 minutes at 190° C. to form thelayer 15μ in thickness.

Method D: The solution (60 parts) which consists of 14 parts ofpartially saponified polyvinyl acetate (average polymerization degree,500; saponification degree, 78.0 mole %) and 86 parts of water, and thewell-ground pigment paste (15 parts) which consists of 14 parts ofabove-mentioned polyvinyl acetate, 56 parts of water and 30 parts of rediron oxide pigment (RO-5097 manufactured by Pfizer) are mixed together.The latex of carboxylated styrene/butadiene copolymer (20 parts) (XD-655manufactured by Dow Chemical Co.) is poured into the resulted solutiongradually under stirring, and 5 parts of teflon power (TL-115 by LNPCo.) is added into the solution. The solution is cast on the plate whichis described in Method B and dried for 2 minutes at 190° C. to form thelayer 20 microns in thickness.

EXAMPLE 2

A mixture of partially saponified polyvinyl acetate (averagepolymerization degree, 500; saponification degree, 82.0 mol %) (35parts), water (30 parts) and Rose bengal (30 ppm of all components byweight) is kneaded in a kneader at 80° to 90° C. for 30 minutes. Then,this mixture is cooled to 60° C. and a mixture of diethylene glycoldimethacrylate (5 parts) B-hydroxyethyl methacrylate (28 parts),hydroquinone (0.1% of total monomer by weight), benzoin iso-propyl ether(0.5 part) and triphenyl phosphine (1.5 parts) is added and stirred for30 minutes. The resulted photopolymerizable composition is cast on theplate which is described in Example 1, Method C. A polyester sheet isplaced thereon and the resulted piled product is passed between tworolls. After cooling, the polyester sheet is peeled off and the plate isdried in a dryer at 75° C. for 40 minutes to form photosensitive layer20 mils in thickness.

EXAMPLE 3

A mixture of partially saponified polyvinyl acetate (averagepolymerization degree, 500; saponification degree 77.0 mol%) (5 parts),partially saponified polyvinyl acetate (average polymerization degree,500; saponification degree 82.0 mol%) (30 parts), water (30 parts) andEosin (30 ppm of all components by weight) is kneaded in a kneader at80° to 90° C. for 30 minutes. Then, this mixture is cooled to 60° C. anda mixture of ethylene glycol dimethacrylate (8 parts), B-hydroxypropylmethacrylate (25 parts), hydro-quinone (0.1% of total monomer byweight), 2,2-dimethoxy-2-phenyl acetophenone (1.0 part) and triphenylphosphine (1.0 part) is added and stirred for 30 minutes. The resultedphotopolymerizable composition is cast on the plate which is describedin Example 1, Method C. The photosensitive plate is obtained by the samemethod as described in Example 2, and the photosensitive layer is 20mils.

EXAMPLE 4

A mixture of partially saponified polyvinyl acetate (averagepolymerization degree, 500; saponification degree 82.0 mol%) (35 parts),water (30 parts) and Rose bengal (30 ppm of all components by weight) iskneaded in a kneader at 80° to 90° C. for 30 minutes. Then, this mixtureis cooled to 60° C. and a mixture of ethylene glycol dimethacrylate (5parts), B-hydroxypropyl methacrylate (28 parts), hydroquinone (0.1% oftotal monomer by weight), benzoin iso-propyl ether (1.0 parts) ando-tolyl diphenyl phosphine (1.0 parts) is added and stirred for 30minutes. The resulted photopolymerizable composition is cast on theplate which is described in Example 1, Method A. The photosensitiveplate is obtained by the same method as described in Example 2, and thephotosensitive layer is 20 mils.

EXAMPLE 5

The photosensitive resin which is obtained by the method in Example 3,is cast on the plate which is described in Example 1, Method D, and thephotosensitive layer is 5 mils after dried.

EXAMPLE 6

A mixture of partially saponified polyvinyl acetate (averagepolymerization degree, 500; saponification degree, 82.0 mol%) (35parts), water (30 parts) and Rose bengal (50 ppm of all components byweight) is kneaded in a kneader at 80° to 90° C. for 30 minutes. Then,this mixture is cooled to 60° C. and a mixture of ethylene glycoldimethacrylate (2 parts), B-hydroxyethyl methacrylate (32 parts),hydroquinone (0.1% of total monomer by weight) and benzoin iso-propylether (1.0 part) is added and stirred for 30 minutes. The resultedphotopolymerizable composition is cast on the plate which is describedin Example 1, Method A. The photosensitive plate is obtained by the samemethod as described in Example 2, and the photosensitive layer is 20mils.

EXAMPLE 7

The photosensitive resin which is obtained by the method in Example 6,is cast on the plate which is described in Example 1, Method C, and thephotosensitive layer is 20 mils after dried.

EXAMPLE 8

The photopolymer plate made according to Example 3 is placed in a vacuumframe and the photopolymerizable surface is brought into contact with aline negative or a half tone negative. It is exposed to a 3,000 watthigh pressure mercury arc for 50 seconds from a distance of 20 inches.After exposure, the negative is stripped from the plate and theunexposed polymer is removed by spray washing with water (temperature,120° F.) under the pressure of 40 psi for 3 minutes. The printing plateis dried at 230° F. for 3 minutes. The printing plates thus preparedshow excellent image quality and long press life when used for directprinting and also are utilizable as original plates for a paper mache.

Table I demonstrates the comparison in quality between thephotosensitive plates with (Examples 3 and 4) and without (Examples 6and 7) phosphine derivatives.

                  TABLE I                                                         ______________________________________                                                   Example                                                                              Example  Example  Example                                              3      4        6        7                                         ______________________________________                                        Exposure Time (sec)                                                                        50       45       180    195                                     Dot Depth (μ)                                                              10% Hi-light 270      260      160    175                                     45% Middle tone                                                                            125      120       50     60                                     90% Shadow   55       50       Plug Dot                                                                             Plug Dot                                Width of Minimum                                                              Remaining Line (μ)                                                                      40       40       140    140                                     Gray Step    17       17        13     13                                     ______________________________________                                    

As shown in Table I above, photopolymer plates made with thephotopolymerizable compositions of the present invention have improveddot depth characteristics in highlight, middle tone and shadow areas.Dot depths that are less than 200 microns in highlight areas, 80 micronsin middle tone areas and 35 microns in shadow areas, (as in the case inExamples 6 and 7) provide dark, unacceptable half tone areas. Moreover,line widths above 50 to 60 microns are generally regarded asunacceptable in newspaper printing applications.

The significance of component selection in the present invention can bedemonstrated through a series of tests in which (1) derivativesemploying Group V elements other than phosphorus are substituted for thephosphine derivatives of the present invention, (2) phosphinederivatives with no phenyl groups are substituted for the preferredphosphine derivatives having 1, 2 or 3 phenyl groups, and (3)halogenated compounds such as pentachlorobenzene are used as anadditional component in the present invention. Table II set forth on thefollowing page shows the results of such tests:

    TABLE II      1 2 3 4 5 6 7 8 9 10 11 12       Formu- PVA (Saponification 5 5 5 5 5 5 5 5 5 5 5 5 lation degree 77.0     mol %)  PVA (Saponification 30 30 30 30 30 30 30 30 30 30 30 30  degree     82.0 mol %)  Water 30 30 30 30 30 30 30 30 30 30 30 30  Eosin 30ppm     30ppm 30ppm 30ppm 30ppm 30ppm 30ppm 30ppm 30ppm 30ppm 30ppm 30ppm     Ethyleneglycol 8 8 8 8 8 8 8 8 8 8 8 8  dimethacrylate  β     Hydroxypropyl 25 25 25 25 25 25 25 25 25 25 25 25  methacrylate Hydroquin     one 0.1% of 0.1% of 0.1% of 0.1% of 0.1% of 0.1% of 0.1% of 0.1% of 0.1%     of 0.1% of 0.1% of 0.1% of   Monomer Monomer Monomer Monomer Monomer     Monomer Monomer Monomer Monomer Monomer Monomer Monomer  2,2-Dimethoxy-2-      1 1 1 1 1 1 1 1 1 1 1 1  phenyl acetophenone  *Activator TPP 1 BDP 1     CPP 1 TBP 1 TPP 1 TPA 1 TPS 1 BPP 1 TPN 1 TEA 1 0 TBP 1  Pentachlorobenze     ne 0 0 0 0 0.5 0 0 0 0 0 0 0.5 Exposure Time (sec.) 50 58 65 120 50 150     135 155 165 180 160 120 Plate Dot 10% Hi-light 270 275 265 150 270 140     155 140 130 120 160 145 Quality Depth 45% Middle Tone 125 120 120 55 120     50 55 50 45 45 50 55  (μ) 90% Shadow 55 55 55 Plug 55 Plug Plug Plug     Plug Plug Plug Plug       Dot  Dot Dot Dot Dot Dot Dot Dot Width of     Minimum 40 50 50 140 40 50 50 80 130 150 140 140 Remaining Line (μ)     Gray Step 17 16 16 13 17 15 15 14 13 11 13 13 Overall Quality Accept-     Accept- Accept- Not Ac- Accept- Not Ac- Not Ac- Not Ac- Not Ac- Not Ac-     Not Ac- Not Ac-  able able able ceptable able ceptable ceptable ceptable     ceptable ceptable ceptable ceptable     *TPP: Triphenylphosphine     BDP: nButyldiphenylphosphine     CPP: Dichlorophenylphosphine     TBP: Trin-butylphosphine     TPA: Triphenylarsine     TPS: Triphenylantimony     BPP: Dibutylphenylbismuthine     TPN: Triphenylamine     TEA: Triethanolamine

Examples 1, 2 and 3 demonstrate that the use of phosphine derivativeshaving 3 (triphenyl phosphine), 2 (n-butyl-diphenyl phosphine) and 1(dichlorophenyl phosphine) phenyl groups in the phosphine activatorcomponent of the present invention all produce printing plates withsatisfactory properties. Surprisingly, however, the degree of acceptancedecreases as the number of phenyl groups is decreased from 3 to 1, e.g.,exposure time increases, dot depth decreases, and gray step valuesdecrease, but all within acceptable limits.

Example 4 demonstrates that the use of a phosphine derivative with nophenyl groups (tri-n-butyl phosphine), with all other parametersremaining the same, does not result in a printing plate havingsatisfactory conditions for newspaper printing applications.

Likewise when derivatives of Group V elements such as nitrogen, arsenic,antimony and bismuth are used in place of phosphorous, the resultantprinting plate has unsatisfactory properties (Examples 6-10).Surprisingly, this is the case even when three phyenl groups are used(tripeanylarsine, triphenylantimony and triphenylamine), or when twophenyl groups are used (dibutylphenylbismithine).

Finally, the presence of a halogenate compound such aspentachlorobenzene neither improves the properties of the resultantplate (compare Examples 1 and 5) nor renders the resultant platesatisfactory when a phosphine derivative with at least one phenyl groupis not present (compare Examples 4 and 12).

It, of course, should be understood that various changes andmodifications to the preferred embodiments described herein will beapparent to those skilled in the art. Such changes and modifications canbe made without departing from the spirit and scope of the presentinvention and without diminishing its attendant advantages. It is,therefore, intended that such changes and modifications be covered bythe following claims.

We claim:
 1. A photosensitive element comprising a substrate and a layerof water developable photopolymerizable composition including:a. about0.1 to 3.0 parts by weight of a monomer component including at least onewater-soluble, monofunctional unsaturated ethylenic monomer, or thecombination of said monofunctional monomer and at least onepolyfunctional unsaturated ethylenic monomer, said monomers capable offorming a polymer by photoinitiated polymerization in the presence of apolymerization initiator activatable by actinic light; b. about 0.1 to3.0 parts by weight of a polymer component including a partiallysaponified, water-soluble, polyvinyl acetate polymer compatible withsaid monomer component containing both acetyl and hydroxy groups, andhaving a polymerization degree of about 300 to 2,000 and asaponification degree of about 65 to 99 mole percent; c. about 0.001 to0.3 parts by weight of a photopolymerization initiator compatible withsaid monomer component and said polyvinyl acetate polymer, andactivatable by actinic light; and, d. about 0.001 to 0.3 parts by weightof an activator including a phosphine derivative described by theformula: ##STR3## wherein any of X, Y and Z are hydrogen, halogen,alkyl, alkoxy, aryl, or vinyl, but not more than one of the X, Y and Zis hydrogen, and wherein at least one of X, Y and Z is aryl.
 2. Aphotopolymerizable composition according to claim 1 wherein saidactivator is selected from the group consisting of triphenyl phosphine,o-tolyl diphenyl phosphine, di-(o-tolyl) phenyl phosphine, tri-(o-tolyl)phosphine, o-methoxyphenyl diphenyl phosphine, o-ethylphenyl diphenylphosphine, and o-chlorophenyl diphenyl phosphine.
 3. The photosensitiveelement of claim 1 wherein said substrate is a chemically ormechanically braded metallic and/or plastic plate.
 4. The photosensitiveelement of claim 1 further having a binder composition carrying aplurality of dispersed particles interposed between said substrate andsaid layer of photopolymerizable composition, said dispersed particlesbeing present in a size and concentration sufficient to create aplurality of protuberances in the background areas of saidphotosensitive element after exposure and development of saidphotopolymerizable composition to create a relief image.
 5. Thephotosensitive element of claim 4 wherein said dispersed particles areselected from the group consisting of (a) glass beads, (b)polytetrafluorethylene and (c) alumina powder and said binder layer isselected from the group consisting of styrene-butadiene copolymers,polyesters, glyoxal, and mixtures of (a), (b) or (c) and partiallyhydrolyzed polyvinyl acetate.
 6. The photosensitive element of claim 4wherein said photopolymerized layer has a thickness between about 0.004and 0.035 inch.